The present invention relates generally to eddy current test methods for measuring the thickness of one or more conductive layers of ferromagnetic and non-ferromagnetic crud and zirconium oxide formed on nuclear fuel rod cladding of nuclear fuel elements, and to eddy current test methods for accurately measuring the thickness of zirconium oxide layer located between the zirconium alloy fuel rod cladding and one or more electrically conductive layers of ferromagnetic and/or non-ferromagnetic crud.
During nuclear reactor operation, nuclear fuel rods which are arranged in each nuclear fuel assembly are submerged in coolant/moderator in the reactor core. In light water reactors using zirconium or Zircaloy cladding tubes for the fuel rods, as a result of the reaction between the water coolant moderator and the zirconium in the cladding tubes, zirconium oxide forms on the fuel rods which can accumulate to a thickness of approximately 200 xcexcm. Because of the adverse effects of the zirconium oxide on the heat transfer from the fuel rod cladding tube to the coolant/moderator, and the thinning of the cladding wall thickness due to metal loss affecting the structural integrity of the cladding, there is a limit on the maximum amount of oxide that is permitted on each fuel rod. Once this limit is reached for a fuel rod, it must be removed from service.
In normal water chemistry reactors (e.g. those that do not have zinc or noble metals added to the water coolant/moderator), typically no or limited amounts of crud is deposited on the fuel rods and standard eddy current lift-off measurement techniques can be used. Such standard techniques measure a parameter (e.g. lift-off vector) produced by the zirconium oxide layer which is then correlated to the thickness of the zirconium oxide layer.
However, the reactor coolant can transport dissolved particles from reactor coolant system components and piping and deposit such dissolved particles on the nuclear fuel rods.
In boiling water reactors (BWR) which have admiralty brass components in steam condensers, and in those BWR and pressurized water reactors (PWR) using zinc injection water chemistry, a ferromagnetic crud also forms on the fuel rods. For example, in reactors using zinc injection chemistry, a layer of zinc spinel (ZnFe4O2) as well as hematite form a tenacious crud layer over the top of the zirconium oxide layer. This crud layer is ferromagnetic due to the presence of iron. Furthermore, the alloy formed by the iron and zinc affects the conductivity and permeability of the crud layer. Measurement of the thickness of both the crud layer and the thickness of the oxide layer are critical for the accurate evaluation of nuclear fuel rod thermal hydraulic performance as well as compliance with fuel rod operating limits, and fuel rod life span.
However, the ferromagnetic material in the crud layer interferes with the standard eddy current lift-off measurement used to determine the thickness of the fuel rod cladding oxide layer. The crud layer having ferromagnetic material has not been measured accurately by the prior art methods which have caused or resulted in an overestimation of the thickness of the cladding oxide layer.
Standard lift-off measurements for measuring the thickness of a corrosion layer or layers employs a probe having a coil(s) of conductive wire which is placed on the surface of the fuel rod. As the coil is lifted off of the surface of the test specimen, the impedance of the coil changes in response to the decreasing effects of the eddy currents produced in the part of the test specimen. The trace the coil produces is called the lift-off vector. When testing nuclear fuel rods, the conductivity of the base or cladding material does not change, which therefore does not effect the retrace path of the lift-off curve. The impedance change is supposed to directly correlate to the distance the eddy current coil is lifted off of the test specimen. As shown in an impedance chart and depicted in FIG. 1, the ferromagnetic material and its associated permeability locus (depicted as that part of FIG. 1 from point A to point C) are in the same basic direction as the lift off curve (depicted as that part of FIG. 1 from point B to point A) for non-magnetic base material (i.e. the fuel rod cladding). A shift in the lift-off locus direction and magnitude is caused by the ferromagnetic crud layer. When this shift in the lift-off locus direction and magnitude is added to the lift-off locus from the non-magnetic base material as is typical for prior art devices, the oxide thickness on the fuel rod is incorrectly determined to be thicker than in actuality by as much as ten times.
The prior art methods of measuring the thickness of the layer of crud containing ferromagnetic material and the zirconium oxide layer on the fuel rod cladding include the necessity of using a correction factor based on anticipated affects of the ferromagnetic crud layer. The correction factor is determined assuming that the effects of the crud layer containing ferromagnetic material are a linear function of thickness. However, this assumption does not represent a correct or even an accurate representation of the actual condition of the crud, which changes thickness, composition, and permeability along the axial length of the fuel rod cladding. Accordingly, this correction factor and assumptions themselves introduce significant aberrations in the actual thickness verses the reported thickness. This correction factor is subtracted from the collected data and only provides a virtual total thickness of the crud layer and the zirconium oxide layer. The thickness of each of the crud layers containing ferromagnetic material and the zirconium oxide layer cannot be determined by such prior art methods.
It would thus be an advantage over prior art devices and methods to accurately measure the thickness of a crud layer containing ferromagnetic material formed on a nuclear fuel rod cladding.
It would thus be a further advantage over prior art devices and methods to accurately measure the thickness of a zirconium oxide layer formed on the surface of nuclear fuel rod cladding.
It would thus be yet a further advantage over prior art devices and methods to accurately measure each of the thicknesses of the crud layer containing ferromagnetic material and of the zirconium oxide layer formed on the surface of nuclear fuel rod cladding.
In accordance with the present invention, a method is provided for determining the thickness of a layer of substantially oxide material on the surface of the cladding of a nuclear fuel rod when said layer of substantially oxide material has an overlying layer of substantially ferromagnetic material, the layer of substantially oxide material having an unknown thickness, comprising the steps of: placing a probe of an eddy current sensor on the surface of the layer of substantially ferromagnetic material, the probe having a coil excited with an alternating current having a first frequency selected for penetrating only into the layer of substantially ferromagnetic material and producing a first complex impedance of said probe representative of the permeability change and thickness change and conductivity change of the layer of substantially ferromagnetic material, the coil being excited with an alternating current having a second frequency selected for penetrating into the layer containing ferromagnetic material and the layer of oxide material and the cladding of the nuclear fuel rod and producing a second complex impedance of said probe representative of the permeability change and thickness change and the conductivity change of the layer containing ferromagnetic material and the measured change in lift-off produced by the layer substantially oxide material; exciting said probe with an alternating current having the first frequency and the second frequency; measuring the first complex impedance and the second complex impedance; phase rotating the first complex impedance so that its polarity is 180xc2x0 from the second complex impedance; and adding the second complex impedance to the phase rotated first complex impedance, said addition producing a resultant complex impedance representative of the thickness of the layer of substantially oxide material.